Tom Allen

This Roadmap covers the status, current challenges, and new science required to address these challenges, for mechanical metamaterials. This is timely, given the sustained increase in mechanical metamaterials research outputs and patents. Sections are grouped according to anomalous properties. These include auxetic (negative Poisson’s ratio), sub-divided into geometric form, flexible and shape morphing, negative thermal expansion, and negative stiffness. Specific sections also cover naturally and biologically inspired metamaterials (characterised by complex multiscale material architectures), nanoscale/molecular metamaterials, and applications of mechanical metamaterials. Key challenges raised throughout relate to scaling up manufacturing for reliable, cost-effective production, particularly for high resolution processes nearing the molecular level, and efficiency of modelling approaches required for these multiscale systems. With opportunities to target their new, useful functionalities, such as the extreme hardness of auxetic materials, or extreme vibration damping via negative stiffness, mechanical metamaterials are contributing to rapid evolution of manufacturing and multiscale modelling.

In pursuit of sustainable and high‐performance materials, this work investigates the potential of Algerian sisal fibers as renewable reinforcements in bio‐epoxy composites. It is proposed that controlled sulfuric acid treatments can alter the fibers' chemical composition and supramolecular structure, thereby enhancing interfacial adhesion and mechanical performance in the resulting composites. Untreated and sulfuric acid‐treated fibers (2, 5, and 8 wt%) were characterized for cellulose, hemicellulose, and lignin contents, crystallinity index, and surface morphology using X‐ray diffraction and scanning electron microscopy. The 5% sulfuric acid treatment gave the best outcome, increasing crystallinity and reducing surface impurities without introducing micro‐cracks. Before composite fabrication, tensile tests on the fibers revealed that untreated sisal exhibited 226.5 MPa and 6.1 GPa for tensile strength and Young's modulus, respectively. The 5% sulfuric acid‐treated fibers reached 421.9 MPa for strength (+86%) and 13.1 GPa for modulus (+115%). Fibers were then incorporated at 10 wt% into a bio‐epoxy matrix to make specimens for tensile, flexural, and Charpy impact tests. The composites reinforced with 5 wt% treated fibers exhibited superior mechanical properties, including higher tensile strength (67.3 MPa, +86%), Young's modulus (2.2 GPa, +115%), flexural strength (109.3 MPa, +156%), and impact resistance (7.6 kJ·m ⁻² , +138%) relative to neat bio‐epoxy. Scanning electron microscopy analysis of fracture surfaces of the composites after impact testing indicated fiber‐matrix cohesion, which would enhance load transfer and give a more homogeneous stress distribution.

Active mechanical metamaterials have the potential to revolutionize material capabilities, by switching between different properties. The active mechanical metamaterial presented here can be remotely programmed to switch between compressive and shear deformation modes that cause stark changes in stiffness. The considered metamaterial uses controlled instabilities to change the buckling mode of electro‐thermally activated beams. The beams form electrical circuits. When selectively charged, they heat (and soften). The effects of manufacturing imperfections are overcome by connecting the beams to a compliant mechanism, allowing reliable control over the compressive buckling modes that cause the stiffness changes. Connection points in the metamaterial resemble a fish‐bone structure, known to exhibit static nonreciprocity, which is actively controllable within the considered metamaterial. As such, it is shown (computationally) that this metamaterial is capable of modulating traction and pressure across a surface. Pressure can be doubled between adjacent unit‐cells while traction can be shielded (i.e., zero) in selected regions. This concept has potential applications in robotic gripper interfaces, and medical devices.

Approximating the stiffness of biological materials can give important insights into how structures deform and when they may fail. Some samples may be too precious to test to destruction, or too fine to position accurately for conventional material testing, which makes it challenging to obtain approximations of material stiffness. Using two‐dimensional scans, non‐destructive bending tests, and finite element (FE) modeling, we show that we can approximate the modulus of elasticity of samples by fitting FE model data to that of experimental bend tests. We demonstrate our protocol on representative whiskers from three species of Carnivorans, including a terrestrial red fox, semi‐aquatic Eurasian otter, and aquatic phocid grey seal. Grey seal whiskers had the highest approximated modulus of elasticity (0.5–19 GPa), followed by Eurasian otter (0.5–13 GPa) and red fox (0.1–1.5 GPa). We suggest that, as in many other biological structures, adaptations in both the shape and material stiffness of the whisker contribute to how it bends when loaded. Specifically, a larger base radius and higher material stiffness both act to increase whisker flexural rigidity in the aquatic grey seal. This protocol has broad applications in comparative biology and provides a way to determine shape and material stiffness information for various flexible specimen types.

Understanding and addressing the needs of diverse demographics is critical for creating sports products or technologies that can enhance the enjoyment, safety, or performance for more inclusive populations. This perspective article has identified opportunities for innovative sports engineering research by considering factors such as sex, socioeconomic background, culture, religion, age, ethnicity, or neurodiversity when developing research studies. The importance of considering diverse study populations, appropriate data collection methods, and ethical considerations to avoid biases is addressed and supported by research.

Statistical shape analysis, or morphometrics, is a technique commonly used in evolutionary biology to summarise a population of samples. Theoretical morphometrics extends the current population of samples into a theoretical space. Using the lawn tennis racket as an example, this paper showcases the potential of morphometrics as a tool for inspiring design concepts for novel sporting goods. It showcases how morphometrics can be applied to summarise the shape of a sample of rackets and applies theoretical morphometrics to systematically present new candidate designs that fall outside of the inputted existing, racket population. The input population was 514 tennis rackets dating back to the origins of the game. The shape analysis was performed on “front-on” silhouette images of the rackets using the R Package Momocs. The outline shape of each racket was reconstructed using the elliptical Fourier transform curve fitting technique. A principal component analysis performed on the reconstructed outlines showed that >90% of the variance in the shape of the rackets was captured by the first two principal components. An evenly spaced grid of theoretical racket shape outlines was then created in a principal component 2 vs. principal component 1 “morphospace”. The limits of this space were then expanded to give a theoretical morphospace that extended beyond the range of the first two principal components for the 514 samples. We propose that the shapes located within such a theoretical morphospace could inspire designers and help them to systematically identify candidates for novel products. Specifically, we suggest that experimenting with wide-angled throats and heads with angled sides might be an interesting starting point for exploring future tennis racket design concepts. The novel technique presented here could be used by a sporting goods brand during the ideation phase of product development to schematically summarise current designs and identify candidates for new ones.

A finite element model was developed for assessing the efficacy of rugby body padding in reducing the risk of sustaining cuts and abrasions. The model was developed to predict the onset of damage to a soft tissue simulant from concentrated impact loading (i.e., stud impact) and compared against a corresponding experiment. The damage modelling techniques involved defining an element deletion criterion, whereby those on the surface of the surrogate were deleted if their maximum principal stress reached a predefined value. Candidate maximum principal stress values for element deletion criteria were identified independently from puncture test simulations on the soft tissue simulant. Experimental impacts with a stud were carried out at three energies (2, 4 and 6 J), at three angular orientations (0°, 15° and 30°) and compared to corresponding simulations. Suitable maximum principal stress values for element deletion criteria settings were first identified for the 4 J impact, selecting the candidates that best matched the experimental results. The same element deletion settings were then applied in simulations at 2 and 6 J and the validity of the model was further assessed (difference < 15% for the force at tear and < 30% for time to tear). The damage modelling techniques presented here could be applied to other skin simulants to assess the onset of skin injuries and the ability of padding to prevent them.

Sports concussions are a public health concern. Improving helmet performance to reduce concussion risk is a key part of the research and development community response. Direct and oblique head impacts with compliant surfaces that cause long-duration moderate or high linear and rotational accelerations are associated with a high rate of clinical diagnoses of concussion. As engineered structures with unusual combinations of properties, mechanical metamaterials are being applied to sports helmets, with the goal of improving impact performance and reducing brain injury risk. Replacing established helmet material (i.e. foam) selection with a metamaterial design approach (structuring material to obtain desired properties) allows the development of near-optimal properties. Objective functions based on an up-to-date understanding of concussion, and helmet testing that is representative of actual sporting collisions and falls, could be applied to topology optimisation regimes, when designing mechanical metamaterials for helmets. Such regimes balance computational efficiency with predictive accuracy, both of which could be improved under high strains and strain rates to allow helmet modifications as knowledge of concussion develops. Researchers could also share mechanical metamaterial data, topologies, and computational models in open, homogenised repositories, to improve the efficiency of their development.

Auxetic closed cell foams, and highly viscoelastic foams, both show potential to improve impact protection. Specifically, auxetics adapt to the shape of impacting bodies, while highly viscoelastic foams stiffen during severe impacts. So, we made auxetic closed cell foam sheets, including those that were infused with (highly viscoelastic) shear thickening gel (STG). We then undertook comparative quasistatic and impact (drop) tests. Quasistatic tests included compression, tension and indentation. Impact tests were with a flat faced impactor at energies of 1, 3 and 5 J, and a 50 mm diameter hemisphere at 1 and 3 J. Poisson’s ratios of the foams were obtained by optical full field strain measurement. An analytical model was used to separate the contribution of the various measured orthotropic properties during the hemispherical impact and indentation tests. The Poisson’s ratios of the converted foams (both with and without STG) were close to zero or marginally negative when measured through thickness. Planar values of Poisson’s ratio (measured in tension) were as low as −0.6. Through thickness Young’s moduli of the converted foams were 0.5 MPa, and planar moduli were ∼12 times higher. The auxetic foams outperformed the unconverted ones during the more severe impacts, exhibiting about half the peak force during the 3 J hemispherical impacts (2.5 vs. 5 kN). The reduction in peak force was related to a measured doubling in indentation resistance for the auxetic foam. The analytical model suggests that 7%–15% of the measured doubling in indentation resistance was due to (negative) Poisson’s ratio. Infusing the auxetic foams with STG caused, at best, a marginal reduction in peak impact force, attributed to low and non-uniform levels of infusion.